index.js

let initialData = { foo: 'bar' };

let App = Elm.embed(Elm.Main, document.getElementById('main'), { setData: initialData });

Main.elm

port setData : Signal Data.Model

incommingData : Signal Action.Action
incommingData =
  Signal.map UpdateData setData


-- App starting

app : StartApp.App Model
app = StartApp.start
  { init = initialModel
  , update = update
  , view = view
  , inputs = [incommingData] }

main : Signal Html
main =
  app.html